are you using flash cs4?
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In CS4, you can insert "property" keyframes for individual properties, wheras in previous versions a keyframe meant a new copy of an instance. This is why the motion editor lets you animate individual properties. This for example lets you control position animation independently from alpha animation, as you can animate each separately.
The context menu you are seeing lets you insert a keyframe for an individual property (or set of properties - scale X and Y, for example). If you want to set a new keyframe for all current properties, select "All" (or press F6). If you want to just animate the position, insert a new position keyframe by selecting Position from that menu. That inserts a keyframe for just the X and Y position properties (this makes a lot more sense by looking in the Motion editor for the selected tween span).
For a guide on the changes between classic tweening (earlier versions of Flash) and CS4, you can see this article here: http://www.adobe.com/devnet/flash/articles/motion_migration_guide.html.